This invention relates to a capacity control device for a compressor suitable for use with an air conditioning system for an automotive vehicle.
Control of cooling capabilities of an air conditioning system used with an automotive vehicle is effected by adjusting the degree of opening of an expansion valve located at an inlet of an evaporator in accordnace with the degree of superheating of a refrigerant at an outlet of the evaporator (the degree of superheating being a function of the temperature of the refrigerant at the outlet of the evaporator and its pressure at the inlet of the evaporator, and varying depending on a thermal load applied to the evaporator), to thereby control the volume of the refrigerant flowing in circulation through the refrigeration cycle.
Generally, the expansion valve is constructed such that it is not fully closed even when the thermal load is low and kept at a predetermined low degree of opening because of the need to return at all times a lubricant to the compressor, the lubricant having flowed out of a compressor to the refrigeration cycle.
Since the compressor of an air conditioning system for an automotive vehicle is rotatably driven by an engine of the vehicle for rotation, the flow rate of the refrigerant flowing through the refrigeration cycle varies in dependence upon the number of revolutions of the engine (i.e. engine speed) even if the degree of opening of the expansion valve is kept constant.
Thus, in the condition that the thermal load is low and expansion valve is kept at a lowest degree of opening, if the engine speed rises as the vehicle is operated at high speed because as of acceleration, ascending a slope or passing another vehicle, the number of revolutions of the compressor would also rise and the flow rate of the refrigerant would exceed a desired level.
As a result, the evaporator would freeze and the heat exchange rate would drop to a very low level, resulting in a malfunctioning of the evaporator. To avoid this phenomenon, it is a usual practice to provide a freezing prevention means for temporarily rendering the compressor inoperative by monitoring the temperature of the air exhausted through the evaporator or the surface temperature of the evaporator itself. However, stopping and reactivating the compressor rotating at high speed would shorten the service life of the compressor and an electromagnetic clutch. If the compressor is reactivated before the balance is redressed between the pressure at the suction side of the compressor and the pressure at its discharge side, the torque required for restarting the compressor would be high and cause a variation to occur in the load applied to the engine, thereby deteriorating the traveling performance of the vehicle. (If this phenomenon occurs when the vehicle ascends a slope or accelerates, the engine might stop.)
To avoid this problem, a proposal has been made in Japanese Patent Laid-Open No. 26969/82 to effect control of the capacity of a compressor by controlling the number of revolutions of the compressor in accordance with the magnitude of a thermal load irrespective of the engine speed.
The proposal may be effective but it uses a variable pulley and means for controlling the variable pulley in accordance with the thermal load to control the number of revolutions of the compressor. This renders the construction complex and causes a rise in cost.